The present invention relates to a mechanical shaft-coupling device for coupling a driving shaft connected to a motor with a driven shaft connected to its load.
It has been observed that in mechanical manipulation by use of rotating tools over-load in the manipulation leads to over-torque given to such tools, which become disfigured and unusable or else broken and scattered thereby menacing the operational safety.
With a view to prevent such a dangerous accident, it is desirable to hold the rotating tool not by the driving shaft directly but by the driven shaft coupled with the driving shaft by a shaft-coupling device, which transmits normal torque under normal load but stops its torque transmission as soon as the revolving tool becomes overloaded, and furthermore it is desirable to make the shaft-coupling device resume its normal function as soon as the revolving tool comes back to the normal load condition.
The desirable shaft-coupling devices of the prior art, such as fluid couplings or electro-magnetic couplings, have apparent disadvantageous points such as their complicated mechanism, troublesome handling and expensiveness. Because of this fact, a shaft-coupling device with comparatively simple mechanism has been called for.
As a means of satisfying the above conditions, there has been used such a mechanical shaft coupling device as resiliently-supported gear clutch type, comprising a clutch disc being equipped with partial concave radial fosses reaching the circumference of the disc and having side walls inclined circumferentially and opened toward the other clutch disc being equipped with corresponding partial projections to engage the fosses, and the projections being supported resiliently by plate springs.
Under normal load given to the driven shaft, the projections are pressed by the spring plates so that the projections engage with the faces for normal torque transmission, while if the driven shaft comes to take an overloaded condition, the axial component force of the torque applied to the inclined sides of the fosses and projections wins over the spring plate's resilient force, whereby the coupled faces and projections disengage to cut the torque transmission.
However, this mechanical shaft-coupling device of the prior art, when the driven shaft is under overload, makes the driven shaft turn idlingly rubbing the clutch disc with considerable pressure equivalent to the repelling power of the spring plate compressed through disengagement of the coupling device, with the result that the tips of the projections and the inclined side walls of the faces sustain rapid frictional wear. This wear, therefore, rapidly changes the shape of inclined contact surface of the projections and faces, the inclined angle becoming less and the torque limit for disengaging the coupling dropping in a short time. Not only the initially-presecribed torque limit can not be maintained as it is, but it also becomes entirely impossible even to transmit the normal torque.